An OFDM receiver adapted to receive orthogonal frequency division multiplexing (OFDM) signals estimates a channel state using pilot signals to correctly decode transmitted data signals. In a time-frequency pilot pattern, pilot signals are scattered along the carrier frequency axis (i.e., in the carrier direction) and along the time axis (i.e., in the symbol direction) at constant intervals. Based upon the pilot signals, symbols and carrier frequencies are recovered by interpolation to estimate the channel characteristic. By dividing the received data symbols by the estimated channel characteristic, the data signal is corrected. To improve the signal receiving performance, it is important to improve channel estimation accuracy.
In general, an adaptive filter is used to estimate the channel characteristic for data signals. Adaptive filters are superior to fixed filters because the filter coefficients can be optimized according to the propagation environment.
FIG. 1 is a conventional schematic diagram of a data signal correction circuit using a typical adaptive filter 108. A received signal, which has been subjected to fast Fourier transform at a fast Fourier transform (FFT) calculator unit 3, is separated at a signal separator unit 11 into a pilot signal and a data signal. At a symbol interpolator unit 12, the pilot signal is recovered in the direction of time by interpolating between pilot symbols. Then, at a carrier interpolator unit 13, the symbol-interpolated signal is further recovered in the direction of frequency by interpolating between pilot carriers using a filter with a passband according to the quantity of multipath delay spread, thereby determining a channel characteristic estimate value of the data part.
A first data signal correction processor unit 14 estimates a transmitted data signal by dividing the received data signal by the channel characteristic estimate value. A hard decision processor unit 15 makes hard decision on the estimated transmitted data signal with respect to constellation points. A channel characteristic calculator unit 16 calculates a channel characteristic estimate value by dividing the received data signal by the post-hard-decision transmitted data signal.
A path detector unit 107 calculates a delay profile by performing inverse fast Fourier transform on the post-hard-decision channel characteristic estimate value, and determines a symbol start position of a FTT window. The path detector unit 107 also calculates a multipath delay quantity from the delay profile, and supplies the calculated multipath delay to a carrier interpolator unit 13.
An adaptive filter 108 carries out adaptive equalization on the post-hard-decision channel characteristic estimate value. The filter coefficients are updated by least mean square (LMS) algorithms. A second data signal correction processor unit 109 divides the received data signal by the adaptively equalized channel characteristic estimate value and outputs transmitted data.
FIG. 2 is a schematic diagram illustrating a conventional adaptive filter 108, in which H(n) denotes a channel characteristic estimate value of carrier number “n”. Channel characteristic estimate values of carrier numbers n−α, . . . , n−1, n+1, . . . , n+α are input to the corresponding taps of the adaptive filter 108 with the filter order of 2*α. Each of the inputted channel characteristic estimate values is multiplied by a corresponding tap coefficient w(n, k), where w(n, k) denotes the tap coefficient of the carrier number k with respect to carrier number “n”. The multiplication results of all the taps are added and output. The difference between the added value and the desired signal d(n)=H(n) is an error, which error is used to calculate updated values of the tap coefficients.
By increasing the number of taps of the adaptive filter 108, the resolution can be increased and channel estimation accuracy can be improved. For example, the filter characteristic with filter order 20 in FIG. 3A can be improved by increasing the filter order to 60 as illustrated in FIG. 3B, in which a steep waveform can be obtained.
However, when increasing the number of taps, the circuit size and electric power consumption also increase. It is desired to improve the filter characteristic without increasing the circuit size.
It has been proposed to selectively perform symbol recovery interpolation according to the multipath delay quantity when time fluctuation of a received signal is great. See, for example, Japanese Laid-open Patent Publication No. 2005-45664.